Keeping supplies flowing has been a thorny problem this year for the Western military. Pakistan this month reopened its territory to NATO transit after a lengthy diplomatic standoff, but the volume of cargo has not reached its former levels.

The NATO force has been making increasing use of northern supply routes, and Wednesday’s attack at a truck stop on a major highway appeared aimed at showing those routes’ vulnerability.

The police chief in Samangan province, Gen. Khalil Andarabi, said the predawn explosion targeted a fuel pump, setting off a conflagration. He said authorities were investigating whether the bomb had been attached to the pump by magnet, or planted nearby.

The attack marked the second major episode of violence in less than a week in Samangan province, north of the capital. A suicide attack on a wedding hall Saturday killed at least 19 people, including the father of the bride, a prominent member of parliament.

While insurgents have been seeking to consolidate their foothold in the north, much of the fighting between NATO troops and militants is concentrated in the country’s east, where the Western military said two service members were killed Wednesday by an improvised explosive device, or IED. No details were disclosed, including the slain troops’ nationalities.

The coalition also said a NATO helicopter crashed early Wednesday in western Afghanistan, normally one of the quietest parts of the country. One service member was seriously hurt, and the cause of the crash was under investigation.

[Updated July 18, 11:08 a.m.: Also Wednesday, British Prime Minister David Cameron came to Afghanistan on an unannounced visit, traveling to volatile Helmand province to meet with British troops and commanders. He told journalists Britain hoped to engage in a gradual troop drawdown rather than a “cliff-edge” pullout when the NATO combat mission ends in 2014.

Britain has about 9,500 troops in Afghanistan, the second-largest contingent after the United States.]